The Federal Communications Commission (FCC) has recently promulgated regulations requiring cellular phone operators to provide the location of emergency callers within 125 meters by the year 2001. One way to comply with the new regulations would be for the mobile phone to determine its own location and transmit location data upon placing a "911" call. However, most phones do not have the ability to determine their location. Since the FCC regulations apply to all phones which are in use as of the effective date of the regulations, this method for determining location will not likely be sufficient by itself to comply with the FCC regulations. Notwithstanding the FCC requirement, mobile positioning is desirable for numerous other applications. For example, mobile positioning could be used for fleet tracking. Police or other law enforcement authorities could use mobile positioning to track criminals. Also, individual users can use mobile positioning to determine their own location.
Another method for determining the location of a mobile station is to take measurements of signals transmitted from the mobile station at three or more points and to use such measurements to solve for the position. Various types of measurements could be used to solve for the position of the mobile station including time of arrival (TOA), time difference of arrival (TDOA), or angle of arrival (AOA). While TDOA and AOA measurements can be performed on any signal transmitted by the mobile station, TOA measurements can only be performed on known signals. The performance of TOA is often superior to TDOA and hence, it is desirable to make measurements on known signals whenever possible.
In the AMPS system, which is an analog system, the length of known signals transmitted by the mobile station is relatively short. For example, in reverse control channel message transmissions, most of the message bits are unknown except the first 48 bits. Because the length of the known signals is small, it is difficult to perform TOA measurements which are preferred for calculating the location of the mobile station.
Accordingly, there is a need for a new method for determining the position of a mobile station from the mobile station's transmissions in the AMPS system.